1. Technical Field of the Invention
The present invention relates generally to a vehicle control system designed to control an output torque of a power train of an automotive vehicle as functions of parameters indicative of operating maneuvers of a vehicle driver and variables controlled by other systems installed in the vehicle.
2. Background Art
Japanese Patent First Publication No. 5-85228 discloses a vehicle control system designed to control an output torque of a power train of automotive vehicles. The vehicle control system is constructed of a plurality of hierarchical elements. The hierarchical elements include a vehicle driver and control units installed in the vehicle. Defined to be the highest in level is a vehicle driver and lower in level are the control units.
Specifically, a command from the vehicle driver of the first hierarchical level is sensed by, for example, a position sensor for an accelerator pedal, a brake pedal, or a steering wheel and then outputted to the control unit of the second hierarchical level. This unit outputs a signal to change dynamic running characteristics of the vehicle to meet the intention of the vehicle driver. For instance, the control unit of the second hierarchical level monitors a driver's effort on the accelerator pedal or the brake pedal to determine a positive or negative acceleration of the vehicle as required by the driver and outputs it to the control unit (i.e., a power train and brake controller) which is defined to be at the third hierarchical level and works to change torque of wheels of the vehicle.
The control unit of the third hierarchical level samples the data inputted thereto which indicates the required acceleration (i.e., the intention of the driver). When the data indicates a request to brake the vehicle, the control unit determines a required degree of deceleration or brake torque to be applied to each wheel. Alternatively, when the data indicates a request to accelerate or keep the vehicle at a constant speed, the control unit determines a required degree of running torque to be outputted from the power train.
Upon the determination of the required brake torque, a control signal indicative thereof to a brake device defined to be at the fourth hierarchical level.
Alternatively, upon the determination of the required running torque, a control signal indicative thereof is outputted to the power train which is defined to be at the fourth hierarchical level. The power train works to determine an output torque of the engine and a gear ratio of a transmission required to achieve the required running torque and output a control signal indicative of the target engine output torque to an engine controller which is defined to be at the fifth hierarchical level and a control signal indicative of the target gear ratio to a transmission controller which is defined to be at the fifth hierarchical level. The running torque required to be outputted from the power train is, thus, converted into a combination of the target engine output torque and the target transmission gear ratio which is so selected as to minimize consumption of energy in the vehicle.
As apparent from the above discussion, the conventional vehicle control system works to provides a target controlled variable from one of the hierarchical elements which is higher in hierarchical level to a lower one to optimize control of the whole system in order to achieve a selected object (e.g., saving of energy consumption). In other words, the system is so designed as to provide an instruction unilaterally from one of the hierarchical elements to another.
The above type of control system, however, encounters the following drawbacks.
Usually, power trains of automotive vehicles have an allowable torque range within which torque is allowed to be outputted depending upon an operating condition of the engine, loads exerted by engine-driven accessories on the engine, and running conditions of the vehicle. Therefore, when the power train controller is given the target output torque by a higher one of the hierarchical elements which is determined without regard to such an allowable torque range, it may result in a difficulty in controlling an operation of the power train correctly.
Additionally, for instance, if a higher one of the hierarchical elements has failed in operation and has instructed the power train controller in error to output a constant torque continuously, it may cause the power train to be placed in an unintended condition, so that an unwanted torque is outputted.